Fix backing up database to S3

The script checks whether the dump file exists to avoid running pg_dump
twice. The check always passes because there is an empty file created by
mktemp therefore pg_dump is never run and the resulting dump is always
an empty file.

Fix the issue by creating an temporary directory where the dump file
will be created by pg_dump so the "dump file exists" check works as
intended.

backup/backup.sh

@@ -1,7 +1,8 @@
 #!/usr/bin/env bash
 set -o errexit -o nounset -o pipefail
 
-readonly dump_file="$(mktemp)"
+readonly dump_dir="$(mktemp --directory)"
+readonly dump_file="${dump_dir}/dump"
 
 function cleanup {
   echo "Deleting dump file $dump_file"